LARP (Laser Activated Remote Phosphor) approaches are being employed in many applications as they may generate much higher luminance than LEDs, and may do so at high power levels. Because of the low étendue of a LARP source, such light sources may also provide highly collimated light from short focal length optics. Generally, LARP sources operate in a transmissive or reflective configuration. As shown in FIG. 1, in a LARP transmissive configuration 100, focused pump light 110 enters a phosphor converter target 120 on one side and the output light 130, both converted 133 and a portion of the pump 135, is collected on the opposite side and collimated light 150 may be emitted from an optical unit 160. Transmissive configurations require a transparent, thermally conductive substrate 140 and some means to retro-reflect the backward generated light from phosphor 120 into the forward direction, such as a dichroic 190. As a result, transmissive configurations 100, while compact, have pump power and intensity limitations which are dictated by the thermal conduction path from the excited region 170 of phosphor 120 to the heat sink to which the substrate 140 is finally mounted. Because phosphor 120 may undergo thermal quenching or loss of quantum efficiency as the local temperature of the phosphor emission region becomes high (above 150-200° C.), the thermal limits on pump power and intensity ultimately limit the total luminous flux and luminance which can be generated by the LARP converter on phosphor 120. Other problems may arise in transmissive configurations 100 including restrictions on the amount of scattering within the phosphor volume, which in turn may limit how well an emission spot is confined, and therefore limit luminance. If scattering in transmissive configurations 100 is increased to improve spot confinement, efficacy decreases because of enhanced backscattering towards the pump. Additional problems with transmissive configuration 100 include significant difficulties finding transparent/translucent but optically non-absorbing bonding materials which may have minimum thermal contact resistance and do not degrade at high temperatures and high blue optical fluxes.
By contrast, in reflective configurations, the focused pump may excite the same surface as the surface which generates the converted light, which may also scatter the pump light. Therefore, the opposite side of the phosphor need only reflect light with minimal loss. Several options are available, including fabrication of mirror surfaces directly onto the phosphor or bonding the phosphor to a high reflective material with a non-absorptive bonding material. The bonding material in a reflective configuration is not limited to transparent adhesives, but may alternatively consist of zinc-oxide filled silicone for example. In the case of an enhanced, metallized mirror applied directly to the phosphor, any high thermal conductivity substrate material may be directly bonded without requirements on optical absorption. For example, an appropriately designed metal mirror may be soldered with thin film metal barrier/solder layer coatings. As a result, reflective LARP configurations may generally withstand greater pump powers and intensities before the phosphor temperature becomes too high. Furthermore, the phosphor may be made with very high scattering to obtain good spot confinement, as high backscattering may actually be beneficial, especially in the case of white conversion where a portion of the pump light may be scattered back in the same direction as the converted light emission. Finally, because bonding materials and substrates may see reduced or no short wavelength pump light, it must only withstand the operating temperatures. In the case of a mirror directly on the phosphor, a large range of high thermal conductivity materials may be used for bonding the mirror to a substrate without regard to optical absorption; therefore, degradation issues can be obviated.
The reflective configuration has two basic variations: 1) use of an external dichroic beam-splitter 220 to pump incident laser light to phosphor 230 mounted on substrate 290 from one optical path 240 and then collect the longer wavelength converted light 280 along another optical path 250 as shown in FIG. 2; and 2) pump the phosphor 320 surface from an off-axis pump beam 310 and then collect both converted light 330 and scattered pump light 340 as shown in FIG. 3.
The configuration in FIG. 2 may be highly optimized and lead to very high collection efficiencies if collimating lenses 260 are chosen carefully. Substrate 290 may be embodied as a heat sink such that the heat sink is a thermal conductor. Additionally, substrate 290 may provide mechanical stability and bonding of phosphor 230 and provide a highly reflective surface. However, this is an expensive configuration and may require a separate optical channel 270 for white light generation.
The alternative reflective configuration 300, FIG. 3 may be far simpler and therefore more compact and less expensive. It may also be well adapted for white light generation; however, off-axis pumping 310 generally requires careful optical design to bring in the focused laser light from a far enough distance so as to permit high collection efficiency of the emitted and scattered blue light 330. In general, spatial scales for good optical pumping and light collection must be small, making the mechanical configuration of all components complicated, especially while trying to take advantage of the high thermal conduction potential of reflective configurations.
Unfortunately, the direct excitation scheme in FIG. 3 may suffer from additional problems which may limit the current applicability. A first issue is that if the optical pumping chain uses either simple spherical/cylindrical optics or even direct excitation from the laser diode, pump beam 310 illuminating phosphor 320 is highly elliptical, implying that the emission spot will also be highly elliptical. In some cases, off-axis pumping 310 exacerbates this problem. Elliptical emission spots are both less desirable for most projection/imaging applications and worsen the thermal problems. A second issue with the direct excitation scheme, especially in applications such as automotive headlights where laser safety is a key issue, i.e. laser light has a specular reflection component 350 which may potentially make its way into the light collection path.
Compact LARP configurations, found for example in automotive applications, have previously only been based on a transmissive configuration. In one case, a ceramic phosphor convertor may be bonded directly to a dichroic coating on a sapphire substrate, similar to FIG. 1. The bonding material is a transparent low temperature glass which is robust to high blue laser fluxes and high phosphor temperatures which are reached during high-intensity pumping. Such approaches can achieve luminances greater than 500-1000 Cd/mm2 with luminous fluxes on the order of 300-700 lumens with a few watts of laser power, and an emission spot on the order a few hundred microns. Alternatively, one may expand the spot size to reach higher total luminances with laser pump powers on the order of 10 W, but not significantly increasing luminance.
A second transmissive approach may include methods in which the phosphor material may be contained within a high reflective scattering medium, such as highly scattering alumina, to both confine the emission spot and aid heat conduction away from the pump spot region. The approaches, while they may potentially withstand higher pump powers and therefore generate greater luminous flux, may also still suffer from difficulties with achieving good spot confinement and therefore a high luminance. Additionally, the approaches may still require a non-absorbing, albeit scattering material e.g. alumina, to provide the necessary heat conduction. Also, the phosphor material embedded in the alumina reflector must not be high scattering otherwise there may exist strong backscattering of either pump or converted light.
Static reflective configurations using the configuration in FIG. 2 are presently used in commercial products, such as a medical fiber illuminator, or projection optics 295. Other configurations may employ phosphor on rotating wheels to achieve even higher pump powers, but are still based on the configuration found in FIG. 2.